Generally, when sewing workpieces by a sewing machine, it is more productive if the sewing operation can be continued without stopping the drive of the sewing machine even at the sewing end of a workpiece by feeding a new workpiece successively. For example, as shown in FIG. 6, using a two-needle double chain stitch sewing machine 5 which has two needles 2, 3 provided parallel to right and left side on the lower end of a needle bar 1 moving vertically, and a looper 4 traversing to the needles from the right side, when forming seams (stitch types 602, 605, 855, etc. of United States Federal Standard) on the workpiece, the thread chain linking between the sewing end of the workpiece and a seam to be made on a next workpiece is smoothly drawn out as the upper cover thread of the seam is supported on the tongue of a needle plate 6.
However, when forming other seams without upper cover thread (stitch types 402, 406, 407, etc. of USFS) on the workpiece, the thread chain consecutive to the sewing end of the workpiece is not supported on the tongue of the needle plate 6 because there is no upper cover thread, and the thread chain is falling into a U-shaped hole flanging the tongue and cut with the traversing looper, and continuous sewing operation is disabled.
To draw out the thread chain smoothly, in a certain case, thread tension devices 7a, 8a, 9a for sewing, and thread tension devices 7b, 8b, 9b for forming thread chain are provided respectively for left and right needle threads 7, 8 and looper thread 9, and threads are passed through both thread tension devices, and the thread tension devices are changed over at the sewing end so that only the former will work when sewing, and only the latter will work when forming thread chain. In other cases, the needle plate, feed dog, and presser foot are improved. Such techniques are disclosed in Japanese Utility Patent Publication No. 39-5653, No. 61-25754, Japanese Patent Publication No. 57-58191 (U.S. Pat. No. 4,186,676), Japanese Laid-open Patent No. 3-23887 (U.S. Pat. No. 5,027,728) and No. 5-300991 (U.S. Pat. application Ser. No. 08/051,481, now U.S. Pat. No. 5,398,627).
For drawing out the thread chain smoothly, it is effective to converge two needle threads into a condensed and slender thread chain, and for this purpose it is better to decrease the tension by the thread tension device 8b for the right needle thread 8, and increase the tension by the thread tension device 9b for the looper thread 9. However, the loosened right needle thread 8 may oscillate irregularly when the needle 3 descends, or droop to be caught on the looper 4 near the under end point of the needle 3, or not be converged sufficiently to form occasional small loops L on the thread chain TC, as shown in FIG. 7, formed consecutively to the sewing end seam S of the workpiece W. Then the small loops L are come out of the thread chain TC, and the loops L may be caught on the feed dog and or the looper, and the thread chain may be cut off.